A WDM signal will typically contain a number of data channels, that is optical signals that have been modulated in such a way as to carry information. Each data channel carries independent information, but non-linear effects in typical transmission media can lead to interference between these channels. These effects include four-wave mixing (FWM) and cross phase modulation (XPM). FWM is a non-linear effect that may occur when two or more signals of different frequencies pass through an optical fibre and which has the effect of generating a signal at a new frequency. These non-linear effects corrupt the signal, limiting the efficacy of the WDM signal in transmitting data from transmitter to receiver. Moreover, they become more significant as the frequency separation between data channels decreases, thereby acting as a limit on the number of data channels (and thus the amount of information) that can be carried by a given WDM signal.
U.S. Pat. No. 6,342,961 describes a system in which adjacent data channels are launched at orthogonal polarisations, in an attempt to mitigate the non-linear effects described above. In this system, two combs of data channels are multiplexed with a broadband orthogonal combiner such that the data channels in the resultant WDM signal alternate between orthogonal polarisation states.
In addition to data channels, WDM systems typically carry a number of loading channels. Loading channels are used within optical systems to saturate line amplifiers correctly for wide band operation and provide optimum channel powers for data carrying channels. Many modern generation systems initially operate with lower channel numbers than intended for the system, and are upgraded to meet traffic capacity demands. “Start of life” systems use loading channels as a substitute for the power of many data channels. These loading channels may be continuous wave (CW) or modulated channels.
Loading channels are multiplexed in with the data channels at the channel level, band level, or aggregate stage of the Submarine Line Terminal Equipment (SLTE).
Current transmission equipment suppliers typically use high power laser combs or filtered Amplified Spontaneous Emission (ASE) noise for loading channels. For lasers, many loading channels are required and/or they have to be modulated to overcome transmission issues. Filtered ASE sources typically have poor stop band rejection and broad linewidths that limit data channel performance.
The loading channel physical attributes, for example power, wavelength, and polarisation state, affect the performance of the data channels and therefore the upgrade strategy. Ideally, loading channels should be depolarised to avoid any polarisation issues arising from the terminal equipment and transmission line. Typically, a small number of loading channels carry most of the power for the link with the data channels acting like sensitive probe signals.
One major issue for a multi-channel repeatered system is Polarisation Dependent Gain (PDG), which is due to an effect known as Polarisation Hole Burning (PHB) whereby the available gain is saturated and depleted for a particular polarisation. If light launched into the loading channels is characterised by a high degree of polarisation, then neighbouring data channels can experience different amounts of gain when propagating through the line amplifiers. The precise level of gain will depend on the extent to which the optical signals propagating in the data channels are aligned in polarisation with light in the loading channels. Over time, system-varying effects change the relative states of polarisation between the channels, leading to a fluctuation in the optical power of the data channel. Even worse, the variation in channel power can affect the Optical Signal-to-Noise Ratio (OSNR) of the channel and also the strength of non-linear effects leading to changes in the Bit Error Rate (BER) performance of the channel.